1. Field of the Invention
The present invention is generally directed to the field of devices which generate color image copies from color image originals and, more particularly to a method and an associated apparatus for calibrating such types of devices.
2. Description of Related Art
It is general practice to describe the results desired for a color hardcopy in terms of the measurable quantity, hereafter referred to as density. The density values are for the three primary colors, red, green and blue.
When an original image is to be copied, it is desired to convert the requested density, for all three primary colors, into the three independent code values, appropriate for the writing device, that would produce the correct densities in the hardcopy. In general, a code value that produces a specific measured density in a neutral tone will not produce the same measure density in a color. For example:
If the combination of red, green, and blue code values EQU X1 Y1 Z1
produce measured red, green, and blue densities EQU R1 G1 B1
then the red code value X1, if used in conjunction with green and blue code values Y2 and Z2, will not in general produce the measure density R1.
Therefore, the proper choice of code value for the desired density of one color depends on the desired densities of the other two colors. In other words, the system is not channel independent. In photographic (film) products, this is caused primarily by inter-layer inter-image effects (IIE) resulting from non-linear chemical interactions among the sensitized layers during chemical development.
A prior art patent of interest is U.S. Pat. No. 3,801,736 entitled "Color Reproduction Method In a Halftone Dot" by T. Kosaka, et al. The method disclosed in that patent operates upon an original having halftone dots of three different colors superimposed on layers of transparent material with predetermined thicknesses. In order to correctly characterize the color of the original, halftone dot equations, which incorporate the thicknesses of the transparent layers as at least one element, are used for providing a correction factor prior to the printing of the dot images onto a copying surface.
Another patent of interest is U.S. Pat. No. 4,060,829 entitled "Method of Color Correction" by T. Sakamoto, wherein color separation signals obtained by scanning a color original are compressed to the color ranges reproducible by printing inks by a color correction circuit and converted to digital signals which are utilized as addressing signals for addressing a memory containing correction signals whereby suitable correction signals can be applied to the device for forming the color image from the original.
In U.S. Pat. No. 4,075,662 entitled "Method and System for Compensating the Non-Linearities in a Reproduction Process" by W. Gall, there is shown a system for reproducing an image scanned from an original by a sensing device and for recording the scanned image to later make a copy of the original image. The patent is directed to a method for compensating for deviations in the gray scale level reproduction of the copied image caused by non-linearities in the process.
In U.S. Pat. No. 4,409,614 entitled "Method for the Reproduction of Originals Which, With Respect to Their Color Content, are Scanned According to a Tristimulus Method" by H. Eichler, there is disclosed the use of a 9-element correction matrix and the conversion of signals into separation densities based upon neutral reproductions.
From the foregoing, it can be appreciated that devices of the type that create hardcopy images from original image media, such as film, inherently contain components which cause the copied image not to correspond in terms of color density on a one to one basis with the original image. The differences and/or variations are caused by a number of factors, for example temperature, different film types, non-linearities in the system, noise generated by the system and, of course, the interaction between the different color channels which causes the human eye to perceive differences between the original and the copy version.
The source of the digital image must be calibrated to provide signals (densities) equal to that which would be observed by physically scanning or measuring the produced hardcopy, therefore, it is desired to determine a relationship (algorithm, function, or 3-dimensional look-up table) that will predict the proper three code values that, when used together, produce the desired three densities. This relationship is valid only for the specific conditions (exposure, photographic processing, environmental conditions, etc.) under which it was derived. For critical work, normal variations in the photographic process and environmental conditions may cause the relationship to fail.